Printers are used to print output from computers or similar type of devices that generate information, onto a recording medium such as paper. Commonly available types of printers include impact printers, laser printers and inkjet printers. The term “inkjet” covers a variety of physical processes and hardware but basically these printers transfer ink from an ink supply to the recording medium in a pattern of fine ink drops. Inkjet printheads produce drops either continuously or on demand. “Continuously” means that a continuous stream of ink drops is created, e.g. by pressurizing the ink supply. “On demand” differs from “continuous” in that ink drops are only ejected from a printhead by manipulation of a physical process to momentarily overcome surface tension forces that keep the ink in the printhead. The ink is held in a nozzle, forming a meniscus. The ink remains in place unless some other force overcomes the surface tension forces that are inherent in the liquid. The most common practice is to suddenly raise the pressure on the ink, ejecting it from the nozzle. One category of drop-on-demand inkjet printheads uses the physical phenomenon of electrostriction, a change in transducer dimension in response to an applied electric field. Electrostriction is strongest in piezoelectric materials and hence these printheads are referred to as piezoelectric printheads. The very small dimensional change of piezoelectric material is harnessed over a large area to generate a volume change that is large enough to squeeze out a drop of ink from a small chamber. A piezoelectric printhead includes a multitude of small ink chambers, arranged in an array, each having an individual nozzle and a percentage of transformable wall area to create the volume changes required to eject an ink drop from the nozzle, in according with electrostriction principles.
The present invention deals with the way ink is supplied to the ink chambers, the conditioning of the ink and the impact of ink conditioning on the operation of an inkjet printhead.
Entrapped Air in the Ink Chambers
It is known that the presence of air bubbles in the ink chamber of a piezoelectric printhead often causes operational failure of the printhead. If air is present in the ink chamber, intended pressure changes resulting from piezoelectric deformation of part of the ink chamber walls will be absorbed by the air, leaving the ink pressure unaffected. The surface tension force of the ink in the nozzle maintains the meniscus and no drops will be ejected from the ink chamber. At the frequencies at which piezoelectric transducers in priezoelectric printhead are operated, i.e. in the khz to Mhz range, not only air bubbles but also dissolved air in the ink can cause operation failure as described above. In the prior art, concepts have been disclosed to avoid air bubbles in the ink chamber by creating an air trap upstream the ink chamber, i.e. prior to the ink entering the ink chamber. Solutions have been proposed in EP-A-0 714 779 and U.S. Pat. No. 4,929,963, both herein incorporated by reference in their entirety for background information only, in the form of air buffers or gas separators that allow air bubbles to rise and evacuate from the ink in an intermediate tank before the ink is supplied to the printhead. In U.S. Pat. No. 5,771,052, herein incorporated by reference in its entirety for background information only, a deaerator tube is disclosed as an internal part of an inkjet printhead. The deaerator tube is an air-permeable, ink-impermeable tubular membrane allowing air to be withdrawn from the ink, through the membrane, via a vacuum source.
Back-Pressure Control at the Nozzle in Fast Scanning Applications
A second point of attention in ink supply systems is the pressure at the nozzle, which is critical to a well-tuned and good operating printhead. Inkjet printheads operate best at a slightly negative nozzle pressure or back pressure. In practice this is often achieved by maintaining a height difference between the free ink surface in a vented ink supply tank and the meniscus in the nozzle. That is, the free ink surface in the vented supply tank is maintained gravimetrically a couple of centimeters below the level of the meniscus in the nozzle. This height difference established a hydrostatic pressure difference to control the back pressure at the nozzle. In reciprocating printhead configurations the ink supply tank is located off axis, i.e. not scanning, because otherwise the lowered position of ink supply tank versus the printhead would interfere with the printing medium transport path. Flexible tubing is used to connect the off axis ink supply tank with the on axis printhead, as disclosed in for example U.S. Pat. No. 4,929,963. During acceleration and deceleration of the printhead, pressure waves are created in the tubes that may significantly disturb the pressure balance at the meniscus and may lead to weeping of the nozzle in the case of a decrease in negative pressure, or breaking of the meniscus in the case of an increase in negative pressure and taking air into the ink channel. Many approaches have been proposed to control the back pressure in reciprocating printhead applications. A back pressure regulation mechanisms in the form of pressure buffers or dampers mounted together with the printhead on the reciprocating carriage are disclosed in EP-A-1 120 257 and U.S. Pat. No. 6,485,137, both herein incorporated by reference in their entirety for background information only. For accelerations and decelerations of the carriage above 1G the response time of these devices is insufficient. In EP-A-1 142 713, herein incorporated by reference in its entirety for background information only, a vented subtank is used. The subtank serves as a local ink reservoir near the printhead and is being filled intermittently from a main tank located off axis. The solution provides a better control of the nozzle back pressure by maintaining a local hydrostatic pressure difference between the free ink surface of the vented subtank and the meniscus.
Degradation with Time of Ink Properties in Printheads (Especially for Inactive Nozzles Over a Longer Period of Time)
Although inkjet ink properties can be well controlled at manufacture and maintained at a reasonable level during transport and storage, some ink properties may degrade when the ink is used in an ink system or maintained in the printhead. For instance, inkjet inks containing VOC's (volatile organic compounds) often suffer from evaporation of some VOC's at the ink meniscus in the nozzle. The viscosity of the ink will change locally in the nozzle, having a negative effect on its jetting properties and potentially leading to a nozzle fall out. The time it takes for an ink to degrade in a way that it leads to a nozzle failure, is often referred to as its latency period. Latency problems often are prevented or recovered by regular maintenance of the nozzles, e.g. by purging the nozzle so that ‘fresh’ ink enters the nozzle. Next to these problems, it has been found that if the retention time of ink in an ink supply system is too long, e.g. during production breaks or overnight, effects like settling of dispersions, auto-curing, etc. may occur. In many cases, reliable operation of an inkjet printer after a production break or production shutdown is only achieved after an extensive startup procedure, including purging of a significant amount of degraded ink retained in the whole or part of the ink supply system to assure that the ink in the ink chambers of the printhead is of good quality and will perform reliably in the printhead. Often these amounts of purged ink are not reusable within the printer setup.
For production type inkjet printing equipment, where high printing speeds and reliability are of the outmost importance, the conditioning of the ink is critical. The solutions proposed in the prior art only partially solve some of the problems described above. Therefore it is an object of the present invention to provide an ink system, incorporated in an inkjet printer, that brings the ink in optimal condition immediately after startup and keeps it in optimal condition during printing.